Economic Geology Vol. 78, 1981, pp. 077-093

Copper Mineralization and Magmatic Hydrothermal Brines in the Rio Pisco Section of the Peruvian Coastal Batholith

R. A. AGAR

Geological Survey Department, Ministry of Mines, P.O. Box R.W. 135, Lusaka, Zambia

Abstract

Cu-Fe-Mo mineralization in the Rio Pisco section of the Peruvian Coastal batholith is spatially associated with the Linga superunit, a suite of monzonitic rocks intruded into Albian volcanics. Petrochemical studies of this superunit indicate emplacement of a differentiation series at a subvolcanic level in the crust. The Cu-Fe-Mo mineralization is located principally in the Albian volcanic envelope and is essentially a low-grade porphyry copper type, although the grade is enhanced where structural controls on the movement of the ore-bearing fluids produced more sulfide-rich vein- and manto-type deposits. Alteration patterns associated with both the mineralization and the Linga superunit suggest a close, predominantly magmatic control on the nature of the hydrothermal fluids. Fluid inclusion studies of quartz from the Linga superunit support this and indicate that emplacement of magmas and mineralization took place at a depth of approximately 8 km. The characteristics of the Linga porphyry copper are compared to those of other such deposits and used to suggest a possible telescoping of geometry of the Andean model of Lowell and Guilbert (1970). Thus, in magmatic hydro- thermal systems like the Linga, the deeper parts of the model are effectively brought nearer the surface.

Introduction

THIS paper describes three types of copper minera]- ization in the Coastal batholith of southern Peru, from the Rio Pisco section (Fig. 1). The three types of mineralization, porphyry, vein or fissure filling, and manto, are all associated with a suite of monzonitic rocks. This relationship, the nature of the deposits and their hydrothermal alteration, as well as fluid inclu- sion studies, are used to demonstrate a magmatic source for both the metals and the hydrothermal fluids.

Geologic Setting The Peruvian Coastal batholith extends for more

than 1,500 km from the border with Ecuador south- ward to the Chilean border (Fig. 1). The batholith ranges in age from Upper Cretaceous to Lower Ter- tiary and for most of its length intrudes Mesozoic and Lower Tertiary rocks (Pitcher, 1978). The batholith north of Lima has been studied extensively and the publications of Pitcher (1978) and Cobbing et al. (in press) provide more than adequate summaries. Re- cent work shows the batholith to be variable along its length and to consist of three segments, the Trujillo, Lima, and Arequipa (Fig. 1; Cobbing et al., 1977). Each segment is characterized by its own particular intrusions which can be grouped into superunits, and each superunit comprises a suite of units which are closely related in space, time, chemistry and petrol- ogy (Cobbing et al., 1977).

The Rio Pisco section is in the Arequipa segment

of the batholith. Although the Lima and Arequipa intrusions are different, the Rio Pisco section shows the modes and levels of emplacement of these two segments of the batholith to be the same (Agar, 1978). Both were emplaced at subvolcanic levels and both exhibit brittle fracturing, cauldron subsidence, and gaseous entrainment phenomena (cf. Bussell et al., 1976).

The intrusions of the Arequipa segment as exposed in the Rio Pisco section can be grouped into four superunits together with one other unit of limited extent (Table 1). The gabbros are very similar to those of the Lima segment, previously described by Regan (1976), Bussell (1975), and Mullan and Bussell (1977). They are variable texturally, mineralogica]ly, and chemically even on outcrop scale. This is due to late- stage net veining by related dioritic and tonalitic magmas accompanied by associated hybridization and amphibolitization (Regan, 1976).

The Incahuasi superunit is a suite of diorites and granodiorites and, like the gabbros, has no associated mineralization. The Linga monzonitic rocks and the Tiabaya tonalite-adamellite superunit, both have as- sociated mineralization (Fig. 2) though only the for- mer is considered here. The youngest unit is the Char- acas monzogranite which has only a limited outcrop and is of relatively minor importance.

The envelope of the batholith in the Pisco valley consists of a series of sedimentary and volcanic rocks ranging in age from uppermost Jurassic to Lower Tertiary. Some gabbros are deformed along with their

677

678 R.A. AGAR

Km 0 300 Km

RIO PISCO SECTION

XArcquipa ( /

/

FIG. 1. The location of the Rio Pisco section in the segmented Peruvian Coastal batholith.

folded Albian envelope, whereas others clearly trun- cate late Albian fold axes. The Lower Tertiary vol- canic rocks of the area are intruded by outlying stocks of the Tiabaya adamellite but lie unconformably over older members of the batholith. Thus, as in the Lima segment, igneous activity commenced in the Albian and continued at least into the early Tertiary (Wilson, 1975, Bussell et al., 1976).

The Linga Superunit The Linga superunit is a suite of monzonitic rocks

ranging from monzogabbro to monzogranite (Table 1, Fig. $) which crop out together on the western margin of the batholith (Figs. 2 and 4). The rocks are intermediate to fine grained and show good igneous textures. In the monzogabbro and monzodiorites, cu- mulose textures are well preserved whereas grano- phyric textures predominate in the monzogranitic units.

Early workers in the Arequipa segment of the

batholith considered the Linga superunit to be a suite of hybrid rocks produced either by the assimilation of early gabbros by later granitic magmas (Stewart, 1968) or by the potash metasomatism of early gabbros by volatile fluids emanating from later granitic mag- mas (Hudson, 1974). However, the presence of the above igneous textures makes such an origin unlikely. Furthermore, petrologically the modal abundances of hornblende and plagioclase, when plotted against those for quartz, reveal a near linear regression which corresponds to the order of emplacement and hints, in the case of plagioclase, at the differentiation of three separate rhythms, the Humay, Auquish, and Rinconada, each of which represents a discreet batch of magma derived from a common differentiating parent (Fig. 5). A quartz-alkali feldspar-plagioclase triangular plot shows the same rhythms (Fig. 13). While the major oxide chemistry of the Linga su- perunit fails to reveal these separate rhythms, all show a strong linear relationship when plotted on a Larsen variation diagram lending further weight to the ar- gument for a primary magmatic origin for the Linga superunit (Fig. 6).

It is important at this point to examine the nature and distribution of quartz in the Linga superunit. In all units it occurs graphically intergrown with ortho- clase, and only in rocks which have undergone po- tassic or sericitic alteration is enlarged or recrystal- lized quartz observed. Modal abundances of quartz vary little within units and in the superunit as a whole increase with decreasing age (Appendix I). These fac- tors, coupled with the above-mentioned petrological and chemical trends, show the quartz to be primary and related to the Linga magmas and not secondary or hydrothermal in origin.

Each unit is emplaced in its own particular pluton. Contacts are, in the main, steep, sharp and clear-cut. The main controls of emplacement are cauldron sub- sidence, stoping and possibly uplift of the roof. The collapsing of elongate blocks with subsequent em- placement of magma around these blocks produced the characteristic long, narrow plutons of the Humay rhythm (Fig. 4). The Auquish rhythm, on the other hand, was emplaced around a central collapsed block of gabbro to produce the Auquish ring complex (Fig. 4).

There is evidence in and around the Auquish ring complex of gaseous entrainment in the form of col- lapse and intrusion breccias (Fig. 11). This, together with the predominance of brittle fracture phenomena during the emplacement of this superunit and the intermediate- to fine-grained granophyric textures of its more acid members, is taken to be indicative of

As used in this paper, rhythm refers to a series of intrusions.

COPPER MINERALIZATION IN THE PERUVIAN COASTAL BATHOLITH 679

TABLE 1. Units of the Coastal Batholith in the Rio Piseo Section

Youngest

Characas monzogranite Medium-grained, pink graphic granite with less than 5 percent biotite and no other marlcs; plagioclase (Ana0); quartz and perthitic orthoclase granophyric

Tiabaya superunit Tiabaya dikes Tiabaya adamellite Tiabaya granodiorite Tiabaya tonalitc

All units except the dikes are coarse grained and leucocratic: dikes are fine and darker; all are characterized by 8:1 prismatic hornblendes and euhedral books of biotite; plagioclase (An_.); orthoclase perthitic, quartz biastic; xenoliths are small, round, and dioritic

Linga superunit Rinconada monzogranite Auquish monzogranite Auquish prophyritic monzonite Auquish monzonite Humay monzonite sheets Humay monzonite Humay quartz-monzodiorite Humay monzodiorite Humay monzogabbro

Suite of variable grain size, medium to fine, characterized by pale green laths of plagioclase (An), dark green anhedral hornblende with pyroxene cores often distributed in clots, rare biotite, low-quartz content (always less than 20%), and salmon-pink, perthitic K- feldspars graphically intergrown with quartz; volcanic xenoliths in swarms at contacts and isolated dioritic xenoliths in the main

Incahuasi superunit Huaytara granodiorite Magocancha granodiorite Incahuasi diorite

Medium to coarse grained with euhedral plagioclase (An) and poikilitic hornblende and biotite; marlcs often occur in clots and hornblende has pyroxene cores; orthoclase perthitic and quartz biastic; xenoliths are usually small, dioritic, and occur in trains

Patap superunit Gabbros A variety of olivine, pyroxene, and hornblende gabbros; net veined and amphibolitized by

related diorites and tonalRes

Oldest

a subvolcanic or hypabyssal level of emplacement (cf. Bussell et al., 1976). Copper-Iron-Molybdenum Mineralization in the

Rio Piseo Section

Cu-Fe-Mo mineralization in the Rio Pisco area has a clear spatial relationship with the Linga superunit (Figs. 2 and 7), although mines are almost universally located within the superunit's volcanic envelope.

Three types of mineralization can be recognized; one of low grade and two in which the grades are en- hanced by structural controls on the migration of ore bearing fluids.

The low-grade ore is disseminated throughout the volcanic envelope of each Linga unit with a geometry like that of a porphyry copper deposit (Fig. 8A, type c). $ulfides of copper, iron, and molybdenum are widespread close to the contact, but the grade is gen-

-* - -- ' 2 Lingo Super- unit 2 Other Intrusive rocks K Volcanic rocks Cu, Pb,Zn0Ag Mine workings Cu,Fe,Mo Nine workings

FIG. 2. The location of mine workings in the Rio Pisco section.

680 lq. ,4. ,4G,41q

A P

Units of Hummy Rhythm ..... Trend of parent mmgmm Units of ^ uqulsh Rhythm ..... Trend of rhythm Rlnconada Ionzogranlte

FIc. $. A quartz (Q)-alkali feldspar (A)-plagioclase (P)-plot of the Linga superunit. Numbers 1-5 refer to position of unit in em- placement sequence of a particular rhythm (age 1-5, oldest to youngest).

erally less than 0.2 percent Cu. Whenever the en- velope consists of an earlier intrusion, the mineral- ization is suppressed with only very narrow chalcopyrite-pyrrhotite-molybdenite veinlets depos- ited along joint surfaces.

Vein deposits located at a distance from the contact show enhanced grade and are common throughout the volcanic envelope. The veins are both parallel and perpendicular to the main Andean trend and reflect the movement of hydrothermal fluids along joint and fault planes. Grades of up to 8 percent copper can be found in major veins, particularly in the heavily faulted Cinco Cruces mining district (Fig. 7). Each vein shows a complex history of ore deposition, often with as many as four or five episodes each followed by disruption and brecciation of the ore and gangue minerals from subsequent fault movements. This ep- isodic mineralization is probably due to the successive intrusions of the Linga units, which were genetically associated with resurgent movements along the faults. Movement of the hydrothermal fluid and the subse- quent deposition of ore occurred in a manner similar to the "seismic pumping" mechanism of Sibson et al. (1972).

The principal ore minerals in these deposits are chalcopyrite, pyrite, bornite, hematite, and molyb- denite, with quartz, calcite, gypsum, azurite, and

malachite the principal gangue minerals. There is very little supergene enrichment and mining opera- tions in the Cinco Cruces area have now largely ceased. The only major mining operation in this area, Mina Eliaria in Quebrada Rio Seco (Fig. 9), has now also closed. Here, both of the above types of miner- alization can be seen, but there is also a third type. A gabbro sill $00 m thick intrudes volcanic rocks and both are in turn intruded by the Rinconada Monzo- granite with a thin wedge of volcanic rocks between the two intrusions (Fig. 9). Within this volcanic wedge, close to the contact with the Rinconada mon- zogranite, is a disseminated type of deposit which is typical of the area as a whole. Mineralized veins ex- tend upward from this disseminated deposit toward the gabbro sill, immediately beneath which are large lenticular bodies of ore (Fig. t0). The gabbro is un- mineralized except for minor traces of copper min- eralization along joint planes, whereas in the volcanics above the sill, veins typical of the area as a whole have been found (Hudson, 1968).

The volcanic rocks both above and below the sill are heavily fractured and jointed; the gabbro is par- ticularly massive and poorly jointed. The lenticular orebodies are located immediately below the gabbro which acts as a ceiling for each lode (Fig. 10). The gabbro is little altered and the ore-gabbro contact is sharp and may be marked by a thin band of actinolite, although normally massive chalcopyrite and pyrrho- rite are found immediately adjacent to the gabbro. The centers of the lodes are always massive chalco- pyrite and pyrrhotite but pass outward to zones where actinolite and then pyrite become important (Fig. t0). The outer margins of the lodes are marked by an assemblage of pyrite, chalcopyrite, apatite, and ac- tinolite. Other minerals present include bornite, mag- netite, and hematite. In addition to actinolite and apatite, the gangue minerals include quartz and cal- cite. The highest grades are found in the center of the lodes where the ore may contain as much as 15 percent copper over distances of up to 5 m.

The geometry of this deposit (i.e., both the zoning and disposition of the ores) suggests that the hydro- thermal fluids responsible for its deposition were ponded or trapped in the heavily fractured, perme- able volcanic rocks by the overlying massive, im- permeable gabbro sill. Philipps (1972) described a similar situation where the impermeability of a thick mudstone unit overlying heavily fractured gray- wackes caused the formation of the mineralized fiats within the graywackes of the Van mine, Montgomery, Mid-Wales.

The distribution of the mineralization in the area as a whole might suggest an exhalative origin for the ores of each type of deposit. For example, all min- eralization occurs in the Albian volcanic envelope

COPPER MINERALIZA TION IN THE PER UVIAN COASTAL BA THOLITH 681

Drift Rinconodo Monzagranite

AUUISH RHYTHM

. A3 Monzagranite i:.:.,' A2 Porphyry fL A1 Ivlonzonite

H UbIAY RHYTHM

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Pampa Cabezo De Toro

HUMAY

Cruces

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Older Intruslw rocks F Volcanic rocks Pampa Chunchonga

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FI(;. 4. The Linga superunit in the Rio Pisco section.

close to contacts with the Linga monzonites which may conceivably have remobilized metals within this envelope during emplacement.

Although there are no mines within the Linga su- perunit itself, traces of copper mineralization occur along joint planes within the various plutons of the superunit. Also, if the source of metals was indeed within the volcanic envelope, one might expect to find similar mineralization wherever the envelope is intruded by the batholith. In the Rio Pisco area, this is not the case and only in the immediate vicinity of the Linga superunit are any of the three types of mineralization observed (Figs. 8 and 6).

Thus, in its simplest form, the Cu-Fe-Mo miner- alization in the Rio Pisco area is a low-grade porphyry copper type associated with the Linga superunit. The ore is disseminated principally in the volcanic en-

velope. Ore grades were enhanced by resurgent movements along faults coupled with episodic hy- drothermal activity and produced the vein- and manto-type deposits, respectively, where structural traps impeded the movement of the ore-bearing fluids and localized the deposition of ores.

Hydrothermal Alteration Associated with the Linga Superunit

Associated with the above mineralization and with each unit of the Linga superunit is a series of alter- ation zones which are typical of those commonly as- sociated with porphyry coppers and described by Lowell and Guilbert (1970) and by Sillitoe (1976). Each unit of the Linga has its own particular series of alteration zones within its own envelope, which may be volcanic, or an earlier member of the Linga

682 l. A. AGAR

% a 25-

20- 15 - lO-

- 5

o-

b 60-

55-

50-

45- -o 35- o

: 30 - n 25-

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Quartz

Huma units Trend of parent magma Auqulsh units Trend of rhythm Klnconada monzogranlte

FIG. 5. Modal abundances of hornblende (a) and plagioclase (b) plotted against those of quartz of the Linga superunit.

superunit, or of the batholith as a whole. Three types of alteration are found. They are potassic, sericitic, and propylitic, and are usually arranged concentri- cally in zones around the parent pluton with the po- tassic zone innermost and the propylitic zone outer- most.

In the potassic zone, secondary biotite replaces hornblende and other marlcs, occasionally in con- junction with chlorite; K-feldspar replaces plagioclase and quartz crystals become enlarged. Magnetite-il- menite intergrowths, common in fresh Linga rocks, are altered to intergrowths of rutile-sphene-magne- rite, not only in this zone but in the others also. Dis- seminated chalcopyrite, pyrrhotite, and molybdenite are also present whenever the envelope is volcanic. Where the envelope is intrusive (e.g., an older Linga unit), the mineralization is suppressed and less evi- dent, although the degree of alteration is apparently no different. The secondary K-feldspar is much red- der in color than the salmon-pink primary K-feldspar typical of fresh Linga rocks. Therefore, the limits of this zone are reasonably recognizable in the field.

The sericitic zone is dominated by sericite which

is secondary after both plagioclase and K-feldspar. The quartz in the rock becomes enlarged, showing later secondary growth, and chlorite replaces horn- blende and biotite. In terms of mineralization, dis- seminated pyrite characterizes this zone, although it also tends to be suppressed wherever the envelope is not volcanic.

The sericite-quartz-chlorite assemblage of the ser- icitic zone grades into the quartz-orthoclase-chlorite- epidote-calcite assemblage of the propylitic zone. Ser- icitization of the K-feldspar becomes negligible and that of plagioclase gives way gradually to alteration to clinozoisite and calcite. Hornblende and biotite are still replaced by chlorite, but epidote becomes more common toward the propylitic zone. The quartz shows no apparent change.

Quite frequently either the potassic or the sericitic zone is missing. In the more basic Linga units, where the potash contents are low (1-2%, Table 2), the as- sociated potassic zone is absent or diminished. In con- trast, the widest zones of potassic alteration are as- sociated with monzogranitic members of the superunit in which the potash contents are much higher ($-4%, Table 2).

Alteration patterns in which the potassic zone is either absent or diminished (Fig. 8B, type d) are con- sidered by Sillitoe (1976) to be indicative of hydro- thermal systems in which the magmatic contribution was minor compared to the meteoric. However, in this case, there appears to be a direct relationship between the amount of KeO in the parent magma and the extent of associated potassic alteration (Table 2). Thus, it may be that the extent of potassic alter- ation is related to the potash content of the parent intrusion.

The sericitic alteration zone is also variable in ex- tent but does not vary according to the chemistry of the parent magma. Widths of only a few meters are typical and frequently the zone is either absent or discontinuous. The restricted development of sericitic alteration zones is common where the wall rock is of a basic nature. This is due to the predominance of biotite over sericite (Guilbert and Lowell, 1974), and may be the case here where the wall rock is gabbroic. However, for the most part, the wall rocks are neither gabbroic nor silica deficient. Sillitoe (1976) believes that the absence of sericitic alteration (Fig. 8B, type c) is more probably due to a relatively small meteoric contribution to the hydrothermal fluids. Thus, the fluids in this case would be largely of magmatic or- igin, a model which is clearly more applicable here.

In detail, the pattern of alteration zones associated with a particular intrusion can be complex. Fre- quently, more than one Linga unit crops out in a small area and each has imposed its own envelope of alteration zones. This may cause a great deal of over-

COPPER MINERALIZATION IN THE PERUVIAN COASTAL BATHOLITH 685

0.12 0.10 0.08 0.06

18--

17--

16-

15--

14-

7-

6-

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4-

3-

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4-

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11-

10-

9-

8-

7-

6-

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-5

MnO 2

AI203

CaO

a20

Total Fe

MgO

I I I I o s lO 1s 20

Larsen Index

FIe. 6. Larsen variation diagrams for major elements in the Linga superunit.

lap resulting in a mineralogically complex series of rocks from the overprinting of alteration.

The tuffisites or hydrothermal breccias associated with the Linga superunit (Fig. 11) are also typical of

porphyry coppers in general and have attendant min- eralization and alteration. These tuffisitic bodies are located in and around the units of the Auquish ring complex (Fig. 11). Usually, they contain finely corn-

684 R.A. AGAR

i.::i Linga Super- unit. Older intrusiw rocks

Volcanic rocks Mine workings

Pampa Cabeza De Toro

Cinco

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Cerros

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HUMAY

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FiG. 7. Cu-Fe-Mo mineralization mine workings in relation to the Linga superunit.

minuted fragments of their host rock and are weakly mineralized (mainly disseminated pyrite but minor amounts of chalcopyrite have also been noted). Al- teration associated with these tuffisites is mainly po- tassic, although argillic zones, in which plagioclase is completely kaolinized, may be found locally.

Alteration patterns associated with the Linga su- perunit are therefore generally typical of porphyry copper deposits, although the variation.in the extents of both the potassic and sericitic alteration zones shows a deviation from conventional models of por- phyry coppers (Hollister, 1975). A greater than nor- mal magmatic control of these phenomena is sug- gested by the positive relationship between the development of potassic alteration and potash content of the parent magma, and the restricted development

of sericitic alteration regardless of both parent magma chemistry and wall-rock composition.

Fluid Inclusion Studies

Quartz in all units of the Linga superunit contains three types of fluid inclusions (Fig. 12). Type 3 in- clusions show a planar relationship to one another and are considered to be secondary and the result of an- nealing along cracks in the quartz (Yermakov, 1965; Roedder, 1967 and 1972). Types i and 2 both occur in isolation and, as it is difficult to conceive of the annealing of a crack to produce a single large inclu- sion, they are thought to be primary (Roedder, 1967).

All three types of fluid inclusions may contain daughter minerals (Fig. 12). These were identified wherever possible using optical techniques (Table 3).

COPPER MINERALIZATION IN THE PERUVIAN COASTAL BATHOLITH 685

a b c

_J'"- Porp. hyritic Intrusive JEJOther Rocks rOCKS

'( Equigranular Intrusive Collapse Breccias rocks

'::..'. .{ Stock work g Disseminated Mineralizaion Alteration Types

K KK Potassic sSs Sericitic pPp Propylitic F FF Fresh

(Affer Sitlilac 1976) FIG. 8. Intrusive situations and styles of porphyry copper min-

eralization (A) and alteration (B).

Type I inclusions are primary, vapor rich and may have up to two daughter minerals; halite and an opaque mineral. Type 2 inclusions are also primary but have only a very small vapor bubble and may have from four to eleven daughter minerals (Fig. 12). The number of daughter minerals in these type 2 inclusions has a positive relationship to the degree of differentiation of their respective Linga units (Table 4). Type $ inclusions are secondary, also have a small vapor bubble and have up to four daughter minerals (Fig. 12).

Microthermometric studies carried out on samples from each Linga unit revealed that all type 1 and some type $ inclusions homogenized completely; the former to a vapor and the latter to a liquid. Type 2 and those type $ inclusions with opaque and bire- fringent daughter minerals did not homogenize com- pletely due to the presence of the above minerals which, being slow to dissolve, would not have main- tained equilibrium with their surrounding fluid. Sev- eral hours are normally needed for such minerals to reach equilibrium (Yermakov, 1965). The results of the microthermometric analyses are tabulated in Ap- pendix II.

Freezing data for type 1 inclusions revealed trapped fluid salinities in the range from 8 to 15 equivalent weight percent NaC1 (Fig. 15). Similar data for two- phase type 3 inclusions gave salinities of 14 to 20 equivalent weight percent NaCI. Heating data for

type 2 and three-phase type $ inclusions gave salin- tries from 20 to 60 equivalent weight percent NaC1, although only very few of the latter had salinities greater than 40 equivalent weight percent NaC1 (Fig. 14).

The primary nature of type 1 and 2 fluid inclusions means that both trapped fluids were present at the time of growth of the quartz. The coexistence of two distinct saline fluids, one gaseous and the other liquid, indicates that as the quartz crystallized, boiling of its parent liquid was taking place (Roedder, 1972). Knowing the salinities of both fluid phases makes it possible to derive the vapor pressure at the time of boiling (Sourtrajan and Kennedy, 1962). In the ease of the Linga superunit, the gaseous phases have a maximum salinity of 15 equivalent weight percent NaC1 giving a minimum vapor pressure of 800 bars (Fig. 15). Similarly, the minimum salinity of the liq- uid phase is 20 equivalent weight percent NaCI giving a maximum vapor pressure of 900 bars (Fig. 15).

By definition, boiling can only take place when the vapor pressure of a liquid is equal to the ambient pressure. In the ease of the Linga magmas, the am- bient pressure may have been wholly or in part due to the lithostatie pressure. Assuming that the ambient pressure was entirely lithostatie and that a mean value of 0.28 kb per km was the lithostatie pressure, the depth of boiling of the Linga magmas would have been between 2.9 and 3.2 km. If the ambient pressure at the time of boiling was only partially lithostatic, then this depth range would be even shallower. Thus, boiling of the Linga magmas as the quartz crystallized occurred at a maximum depth of 3.2 km. The textural nature and distribution of quartz in the Linga su- perunit point to a primary magmatic origin and deny growth from secondary hydrothermal fluids (see above). The fluid inclusion studies indicate that the late magmatic fluids from which the quartz grew boiled during crystallization. This boiling produced two distinct saline fluids. From the nature of the fluid inclusion daughter minerals, the most saline of these fluids contained both the alkali chlorides considered necessary for the transport of ores (Barnes and Cza- manske, 1967) and the ores themselves. Migration of this fluid out from each successive Linga pluton would, on subsequent cooling and precipitation, have given rise to porphyry-type mineralization and to fluids of lower salinity such as those represented in the type 3 inclusions.

Summary and Conclusions The Cu-Fe-Mo mineralization of the Rio Pisco area

is fundamentally a porphyry copper type which has been modified locally to produce small deposits of a higher grade. A certain amount of mineralization and attendant hydrothermal alteration took place with the

686 R.A. AGAR

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FI;. 9. The geology of the Quebrada Rio Seco district.

intrusion of each successive Linga unit. Each unit appears to have produced its own hydrothermal fluids to form their respective deposits. An external source for the metals is discounted in view of the presence of mineralization within the superunit itself and the lack of similar mineralization wherever else the same volcanic rocks are intruded by other members of the batholith.

From the distribution of alteration patterns asso- ciated with both the Linga superunit and the min- eralization, the restricted development of sericitic alteration is taken to indicate the predominance of magmatic rather than meteoric fluids in the hydro- thermal system at this level (Sillitoe, 1976). The pos- itive relationship between the development of the potassic zone and the K.O content of the parent in-

trusion further suggests a close magmatic control of the hydrothermal fluids.

Both minerals and chloride salts are abundant in primary fluid inclusions within quartz of all the Linga units. More importantly, the positive relationship be- tween the number of daughter minerals (hence the complexity of the fluid) within these inclusions and the degree of differentiation of the Linga unit makes an external or meteoric source impracticable. The idea of the hydrothermal fluids being drawn into the pluton by convective circulation of ground water as proposed by Phillips (1978) cannot be applied here.

In such a model, the hydrothermal fluids for each of the nine Linga units would have been derived ini- tially from fresh ground waters of more or less the same salinity. The observed increase in salinity with

COPPER MINERALIZA TION IN THE PER UVIAN COASTAL BATHOLITH 687

T 5m

GABBRO

VOLCANIC ROCKS

'v V

;py-p) py-pyrr- oct

py-oct-py .. .... )y-op

vein deposits .py-

Disseminoted ,py- ))

Limd of )otossic

!

\'," RINCONADA 1,, MONZOGRANITE

FIG. 10. Schematic section through the Mina Eliana ore deposit, Quebrada Rio Seco. Cpy, chalcopyrite; pyrr, pyrrhotite; py, pyrite; act, actinolite; ap, apatite; mo, molybdenite; bo, bornitc.

time could have been brought about by the same meteoric fluid leaching a greater amount of salts and metals from later differentiates purely as a function of the greater concentration of such ions in the more differentiated and fractionated rocks. However, if

TABLE 2. Potassic Alteration Zones in Relation to the K20 Content of Linga Magmas

Linga unit

Maximum width of potassic K20 content

alteration zone of Linga unit

H, Humay monzogabbro 10 1.19% H2, Humay monzodiorite 100 2.38% Ha, Humay quartz monzodiorite unknown 2.05% Ha, Humay monzonite 600 3.79% A, Auquish monzonite 200 2.07% A2, Auquish porphyritic monzonite 900 4.08% As, Auquish monzogranite 1,500 unknown

Abbreviations: H = Humay rhythm, A = Auquish rhythm. Sub- script numbers 1-4 refer to position of unit in emplacement se- quence of a particular rhythm (age 1-5, oldest to youngest)

this were the case, the fluid inclusions preserved would not be primary and their host rock would not be fresh and unaltered. The increase in salinity might also be brought about if the same hydrothermal fluids were in continual circulation throughout the em- placement of the Linga superunit, thus undergoing boiling and further concentration a total of nine times. In this instance, however, one might reasonably expect to find secondary fluid inclusions within the more basic Linga units to show a higher salinity than the primary fluids trapped in the same rocks. This is not the case. Thus, it is more likely that the observed hydrothermal brines are directly of magmatic origin.

Furthermore, the primary igneous texture dis- played by quartz in all the units, the uniformity of modal abundance of quartz in each particular unit, the clear relationships of beth modal abundance of quartz and silica content of the Linga units to time, and the total lack of textural evidence for secondary growth of quartz demonstrate conclusively a mag- matic origin for the Linga quartz. It follows, there- fore, that the fluid from which the quartz grew was

688 R.A. AGAR

Auquish Monzon ire.::, ..... R Gabbro :::.'.. '.... ' ":' '" '" ' '' '" ' "' "" M ine work ngs :')':':" '? Envelope ':. '.?.ii.

.? /-

TABLE . Daughter Minerals from Fluid Inclusions of the Linga Superunit

Daughter mineral Physical and optical properties

Halite

Sylvite

Hematite

x ..:.: :i :i' x :::::i:

Calcite

Unknown A

FIc. 11. The Auquish rhythm and its associated breccias.

Unknown B

Unknown C

Cubic, isotropic, refractive index close to that of quartz

Cubic, isotropic, refractive index less than quartz, dissolves at a lower temperature than halite

Hexagonal platelets, red to opaque; does not dissolve

Prismatic, colorless, refractive index greater than quartz, birefringent, length slow in orientation of least birefringence, does not dissolve

Rhombic, high relief, refractive index greater than quartz, birefringent, did not dissolve

Small, amorphous, isotropic, did not dissolve

Narrow birefringent rod or speck, did not dissolve

Cubic opaque, probably a sulfide, did not dissolve

Unknown D Cubic opaque, smaller than C, did not dissolve

Three other unknowns are isotropic and occur as small specks; two other unknowns occur as birefringent specks; and one unknown is an amorphous opaque

magmatic. This fluid boiled during crystallization of the quartz and gave rise to a low-salinity gaseous phase and a high-salinity liquid phase. These mag- matically derived hydrothermal brines, already ore- bearing, were, on migration out from the parent in- trusions, responsible for the Linga mineralization.

The fluid inclusion studies confirm the subvolcanic level of emplacement indicated by the textures of the superunit and its associated brittle fracture and gas- eous entrainment phenomena. A depth of around 13

km compares closely to that obtained by Roedder (1971) for the porphyry deposit at Bingham. The al- teration halo (Fig. 16b) of the Linga system, however, is more akin to that of deeper deposits such as Wood- stock (Fig. 16a). Although depth determinations for the Woodstock and neighboring deposits are unreli- able, they are still considered to be representative of the deeper roots of porphyry copper systems (Hollis- ter et al., 1974). It may be, therefore, that magmatic hydrothermal systems in porphyry coppers (such as

o 1O.m TYPE DISTRIBUTION NUMBER OF DAUG. HTE

MINERALS

I Gos_rch Omame$ In isolation 0 - 2 lmy mies In 5ion 4-11

5Ollnlty 5ecle5 Plon6r 0 - 4

FIG. 12. Fluid inclusions in the Linga superunit. V, vapor; L, liquid; H, halite; S, sylvite; A, anhydrite; He, hematite; I, ores.

COPPER MINERALIZATION IN THE PERUVIAN COASTAL BATHOLITH 689

4. Daughter Minerals in Primary Fluid Inclusions in Relation to the Age of Respective Linga Units

Maximum number of daughter

Linga unit minerals

H, Humay monzogabbro 5 H2, Humay monzodiorite 6 Ha, Humay quartz monzodiorite 6 Ha, Humay monzonite 8 Hs, Humay monzonite sheets 8 A, Auquish monzonite 8 A2, Auquish porphyritic monzonite 10 As, Auquish monzogranite 11

Taken from ten inclusions per section with sections from eight rocks per unit (i.e., 80 separate inclusions per unit)

the Linga deposit) produce a telescoping effect in their patterns of mineralization and alteration, effec- tively bringing the deeper parts of the porphyry cop- per system nearer to the surface (Fig. 16).

Hollister (1975) defines two fundamental models of porphyry copper deposits, each with its own dis- tinct petrography and alteration halo. One such model, the diorite model is typical of island-arc en- vironments and is characterized the by-product gold and by the lack of sericitic alteration. The Andean model of Lowell and Gullbert (1970) on the other hand is characterized by the by-product molybdenum and the presence of all three alteration zones. In spite of the restricted development of the sericitic zone in the Linga porphyry copper deposit, it is, particularly with its associated molybdenum, more akin to the Andean model of Lowell and Gullbert which is now widely considered to be typical of destructive con- tinental margins (Mitchell and Garson, 1976).

Acknowledgments This work is part of a larger research project within

the Peruvian Coastal Batholith Research Programme

SOLUTION NaCI & SOLUTION

ICE & NaC 12H20 -30 ! I ! I I I I J I I 1

0 5 10 15 20 25 30 60 65 100 Wt% NaCI -

FIG. 13. Salinity ranges of two-phase inclusions from freezing data. A-B, range of type 1 inclusions; C-D, range of type $ inclu- sions (two-phase).

HO

FIc. 14. Salinities of type 2 and $ inclusions with two or more daughter minerals from heating data. O, type 2 inclusions; I, type $ inclusions with more than one daughter mineral; ..... , differ- entiation trend of hydrothermal fluids.

carried out jointly by the University of Liverpool (England), Institute of Geological Sciences (London), and Instituto Geologico y Mineria (Peru). The re- search project itself was based on the regional map- ping of the Coastal batholith in the Rio Pisco area and was financed by the British Ministry for Overseas Development. The author would like to thank the staff of Cobre Sur S. A. (Eliana and Auquish mines) for their hospitality and permission to review their

iaaa

t

500

Q_

liquid

....... Critical curv

" "..:"::..' ::::::::! 1 0 i I I I I I II I.'1 :f.'.,.v.v.. I

0.001 0.01 0.1 1 5 10 50 100

NaCI (Wt. %)

J-J Gas compositions, type 1 inclusions "':'"..:i Liquid .... 2 ,,

FIG. 15. Coexisting gas and liquid compositions relative to pres- sure (after Sourirajan and Kennedy, 1962).

\ \\ / // G,ngham (Peru) \ I /., ., V /

+ + ++ { Iwfoundlend) FRESH Cotheort I I ( Noine ) euATz HONZONIT + '++ ariner +++++++ (Nova Scotia) +++++++

FIG. 16. Schematic representation of porphyry copper alteration patterns relative to depth showing (a) meteoric hydrothermal systems and (b) the Linga magmatic system.

workings and to publish the resulting ideas. Finally, thanks are due to colleagues at the University of Liv- erpool and to Drs. R. Prasad and A. Gunatilaka for their encouragement and their comments on the manuscript. June 1, 1979; August 23, 1980

REFERENCES

Agar, R. A., 1978, The Peruvian Coastal batholith, its monzonitic rocks and their related mineralization: Unpub. Ph.D. thesis, Liv- erpool Univ., 261 p.

Barnes, H. L., and Czamanske, G. K., 1967, Solubilities and trans- port of ore minerals, in Barnes, H. L., ed., Geochemistry of hydrothermal ore deposits: New York, Holt, Rhinehart and Win- ston, Inc., p. 286--388.

Bussell, M. A., 1975, The structural evolution of the Coastal batho- lith in the provinces of Ancash and Lima, Peru: Unpub. Ph.D. thesis, Liverpool Univ., 875 p.

Bussell, M. A., Pitcher, W. S., and Wilson, P. A., 1976, Ring com- plexes of the Peruvian Coastal batholith; a long standing sub- volcanic regime; Canadian Jour. Earth Sci., v. 13, p. 1020-1030.

Cobbing, E. J., Pitcher, W. S., and Taylor, W. P., 1977, Segments and super units in the Coastal batholith of Peru: Jour. Geology, v. 85, p. 625-631.

Cobbing, E. J., Baldock, J., McCourt, W. J., Pitcher, W. S., Taylor, W. P., and Wilson, J. J., in press, The geology of the west cor- dillera of North Peru: London, Inst. Geol. Sci. Overseas Div., Mern.

Guilbert, J. M., and Lowell, J. D., 1974, Variations in zoning pat- terns in porphyry ore deposits: CIM Trans., v. 77, p. 105-115.

Hollister, V. F., 1975, An apprisal of the nature and source of porphyry copper deposits: Minerals Sci. Eng., v. 7, p. 225-233.

Hollister, V. F., Potter, R. R., and Barker, A. L., 1974, Porphyry- type deposits of the Appalachian Orogen: ECON. GEOL., v. 69, p. 618-630.

Hudson, C., 1968, Mina Eliana (Rio Seco), estudio geologico: Tesis Bach., Univ. Nac. de ingeneria, Lima, 15 p.

1974, Metallogenesis as related to crustal evolution in south- west and central Peru: Unpub. Ph.D. thesis, Liverpool Univ., 484 p.

Lowell, J. D., and Guilbert, J. M., 1970, Lateral and vertical al- teration-mineralization zoning in porphyry ore deposits: ECON. GEOL., v. 65, p. 373-408.

Mitchell, A. H. G., and Garson, N. S., 1976, Mineralization at plate boundaries: Minerals Sci. Eng., v. 8, p. 129-169.

Mullan, H. S., and Bussell, M. A., 1977, The basic rock series in batholithic associations: Geology Mag., v. 114, p. 337-359.

Phillips, W. J., 1972, Hydraulic fracturing and mineralization: Geol. Soc. London Jour., v. 128, p. 337-359.

-- 1973, Mechanical effects of retrograde boiling and its prob- able importance in the formation of some porphyry ore deposits: Inst. Mining Metallurgy Trans., v. 821, sec. B, p. B90-B98.

Pitcher, W. S., 1978, The anatomy of a batholith: Geol. Soc. London Jour., v. 135, p. 157-180.

Regan, P. F., 1976, Mafic plutonic rocks of the Coastal Andean batholith, Peru: Unpub. Ph.D. thesis, Liverpool Univ., 352 p.

Roedrier, E, 1967, Fluid inclusions as samples of ore fluids, in Barnes, H. L., ed., Geochemistry of hydrothermal ore deposits: New York, Holt, Rhinehart and Winston, Inc., p. 515-574.

-- 1971, Fluid inclusion studies on the porphyry-type ore de- posits at Bingham, Utah, Butte, Montana, and Climax, Colorado: ECON. GEOL., v. 66, p. 98-120.

-- 1972, Composition of fluid inclusions: U.S. Geol. Survey Prof. Paper 440-JJ, 164 p.

Sibson, R. H., Moore, J. M., and Rankin, A. H., 1975, Seismic pumping: A hydrothermal transport mechanism: Geol. Soc. Lon- don Jour., v. 131, p. 685-659.

Sillitoe, R. H., 1976, A reconnaissance of the Mexican porphyry copper belt: Inst. Mining Metallurgy Trans., v. 85, p. B170-B189.

Sourirajan, S., and Kennedy, G. C., 1962, The system H20-NaCI at elevated temperatures and pressures: Am. Jour. Sci., v. 260, p. 115-141.

Stewart, J. w., 1968, in Garcia, W., Geologia de los cuadrangulos de Mollendo y la Joya: Peru, Servicio Geologia Mineria, Bol. 19, 98p.

Wilson, P. A., 1975, K-At age studies in Peru with particular ref- erence to the Coastal batholith: Unpub. Ph.D. thesis, Liverpool Univ., 299 p.

Yermakow, N. P., 1965, Research on the nature of mineral forming solutions with special reference to data from fluid inclusions: Oxford, London and New York, Pergamon Press, 743 p.

COPPER MINERALIZATION IN THE PERUVIAN COASTAL BATHOLITH 691

APPENDIX I

Modal Abundances of Plagioelase (P), Quartz (Q), Orthoelase (K), Hornblende (H), Pyroxene (Px), Biotite (B), Ores (O), and Accessory Minerals (A) in the Linga superunit

Specimen Unit P Q K H Px B O A

49846 Hi 60.4 5.9 10.1 18.9 tr tr 4.7 49889 H 58.8 5.5 18.0 28.8 tr tr 8.9 49898 H 68.1 6.7 6.7 20.2 tr 1.8 1.5 49890 Ha 58.9 5.9 11.6 22.1 2.8 tr 4.8 51878 Ha 57.9 5.7 18.9 18.4 tr 4.0 51879 Ha 59.4 6.8 17.5 12.2 tr 4.5 49860 Ha 48.2 11.4 17.8 14.7 tr 4.5 8.1 tr 49870 Ha 59.2 12.7 11.7 9.0 tr 4.2 2.9 tr 51880 Ha 54.2 11.0 18.7 17.2 4.2 2.6 tr 51844 H4 44.7 18.0 22.9 15.8 tr 2.1 2.1 51858 H4 45.9 13.8 20.2 18.5 tr 0.9 tr 51870 H4 89.6 18.5 80.7 12.7 tr 2.2 1.8 51816 H5 84.9 18.0 80.6 11.8 2.0 2.7 51842 H5 89.2 17.8 26.8 18.6 1.4 1.7 51858 Hs 87.5 17.6 80.9 10.8 2.1 1.6 49448 A 51.8 16.2 16.9 11.4 tr 8.4 tr 49458 A 47.6 15.9 18.0 14.6 1.5 2.4 tr 51812 A 48.8 15.9 16.7 20.1 2.8 1.5 tr 51872 A2 29.2 26.4 81.1 9.4 0.5 1.9 1.5 49445 As 22.2 29.0 40.4 0.7 5.0 1.9 0.8 49455 As 18.8 82.2 46.0 tr 1.6 1.4 tr 51729 As 10.8 85.8 50.4 tr 0.8 2.0 1.0 51856 R 20.6 83.7 34.2 4.0 7.4 tr tr

Tr = trace Unit abbreviations: H = Humay rhythm, A -- Auquish rhythm, R = Rineonada monzogranite. Subscript number refers to position

of unit in order of emplaeement of rhythm (age 1-5, oldest to youngest) (see Fig. 4)

APPENDIX II

Microthermometric Data for the Fluid Inclusions of the Linga Superunit

Unit Type D Ho V L Ha Sy F

Hi

H2

585+ 535+ 585+ 585+ 580+ 580+ 277.1 de 126.6 869.4 490.9 549.6 210.6 221.0

550+ 550+ 550+ 550+ 560+ 560+ 560+ 560+

150.9 154.5 168.6

182.5

182.2 126.6 869.4 490.9 549.6 192.1 180.6

? 560+ 150.9 154.5 168.6 182.5

585+ -5.1 585+ -8.6 535+ 585+ 580+ - 11.2 580+

55O+ 55O+ 55O+ 55O+ 560+ 560+

210.6 221.0

560+ 560+

560+ 195.5

-15.1

-12.4

-10.6

-14.8 -16.2

692 a.A. AGAa

APPENDIX II--(Continued)

Unit Type D Ho V L Ha Sy F

H3

H4

H5

Ai

A2

3 0 192.3 192.3 $ 0 266.6 266.6 3 0 457.9 457.9

1 0 487.2 1 0 518.9 2 5 550+ 309.6 2 5 510+ 346.9 2 5 510. Odc 248.1 3 0 145.0 145.0 3 0 192.1 192.1 3 0 206.9 206.9 3 0 249.0 249.0 3 0 271.2 271.2 3 1 168.9 119.2 $ 1 182.6 123.8

1 0 506.7 2 6 550dc 317.5 $ 0 101.2 101.2 $ 0 108.4 108.4 3 0 147.4 147.4 3 0 149.7 149.7 3 0 151.5 151.5 3 1 179.4 127.7 3 1 217.3 197.4 $ 1 343.1 175.1 3 1 550+ 238.5 $ 2 440dc 182.8

1 0 565+ 1 0 565+ 2 4 550+ 284.9 2 5 550+ 215.5 3 0 158.9 158.9 $ 0 163.1 163.1 3 1 165.1 162.4 3 1 171.3 137.2 8 1 208.7 194.5

1 0 460.2 1 0 526.0 1 0 55O+ 1 0 55O+ 2 4 560+ 333.7 2 5 560+ 511.3 2 5 560+ 283.7 $ 0 170.0 170.0 3 0 420.1 420.1 $ 0 446.2 446.2 3 1 272.5 272.5 3 1 307.4 307.4 3 1 312.8 312.8

533.3 560+ 560+ 471.1dc 239.9 193.2 193.2 201.1 201.1 209.6 209.6 345.0 345.0 370.3 370.3 428.8 428.8 221.4 212.1 280.8 216.5

487.2 518.9

506.7

565+ 565+

460.2 526.0 55O+ 55O+

533.3 560+ 560+

55O+ 417.8 510+

168.9 182.6

344.4

179.4 217.3 848.1 550+ 44O+

55O+ 55O+

165.1 171.3 208.7

560+ 560+ 560+

137.6 185.9 242.1

471.1+

221.4 230.8

264.7 323.7 238.3

248.4

225.0

242.5 258.9

323.6 190.4 234.1

257.8

-16.4

-9.5

-17.0 -13.7

-7.8

-46.2

-27.8

-9.5

-16.7

-13.1

-14.1

-6.9

-14.5 -12.2 -15.2

COPPER MINERALIZATION IN THE PERUVIAN COASTAL BATHOLITH 6913

APPENDIX II--(Continued)

Unit Type D Ho V L Ha Sy F

A3

$ 1 252.4 190.8 252.4 $ 1 259.9 215.$ 259.9 $ 1 262.7 262.7 192.9

2 7 518+dc 277.2 518+ 2 5 470+dc $27.0 470+ 2 5 555+ $52.$ 554.8 2 5 550+ 420.2 329.8 $ 0 140.5 140.5 $ 0 150.2 150.2 $ 0 172.0 172.0 3 0 182.8 182.8 $ 0 189.1 189.1 3 0 325.7 325.7 3 0 335.8 335.8 3 1 320.9 320.9 269.8 3 1 327.4 327.4 254.6 3 1 329.2 329.2 246.0 3 1 550+ 481.8 550+

518+ 266.2 468.0 187.7

-14.5 -15.$ -14.9

Abbreviations: D -- number of daughter minerals, Ho = homogenization temperature, V = temperature at which vapor disappears, L = temperature at which liquid disappears, Ha = halite dissolution temperature, Sy -- sylvite dissolution temperature, F -- freezing temperature, dc = decrepitation temperature; all temperatures in C.